The invention relates to a system for controlling the ambient air exchange rate of a refrigerated container to control the composition of the conditioned space air, and more particularly the invention relates to a system and method for controlling the composition of the conditioned space air by automatically and precisely setting and controlling the ambient air exchange rate of refrigerated container by comparing a predetermined set point value for a container operating parameter with a sensed value for the operating parameter, and if the sensed value is outside the set point value, automatically adjusting the air exchange rate to obtain the conditioned space air concentration required to preserve the container cargo.
Containers are loaded with perishable cargo such as fruits, vegetables, and flowers for example, and the loaded containers are then placed on ships which bring the perishable cargo to its destination a number of days after the cargo has been loaded in the container. When transporting a temperature controlled load of perishable cargo, the cargo is subject to degradation due to varying concentration levels of common gasses in the conditioned space defined by the transport container. Fresh air exchange with the conditioned space air is used to help prolong the life of the cargo transported in the conditioned space. The commonly present conditioned space gasses include nitrogen, carbon dioxide, and ethylene. Some mixtures of these gasses are beneficial and promote longevity and freshness of the cargo, while other mixtures of these gasses are harmful to the cargo and can reduce freshness and spoil the cargo.
Some of the common conditioned space gasses are actively produced by the respiration of the cargo. Upon loading, the concentration of the conditioned space gasses is acceptable. However, over time while in transit, the cargo respirates and as the gasses are emitted by the cargo, the gas concentration levels change. Depending on the nature of the composition of the conditioned space air, the change in gas concentration could reduce the freshness of the cargo and ultimately result in cargo spoilage. As a result, the concentrations of the common gases in the container conditioned space must be closely monitored and controlled during shipment to prevent the production of harmful gas concentrations.
The typical method for controlling the container concentrations of common gasses is by manually adjusting the position of a fresh air exchange door which, when opened, permits outside ambient air to enter the container and supplant the conditioned space air. After determining the fresh air exchange rate required to maintain the desired concentration of the common gasses in the container, the exchange door is manually opened as required to the door position that will produce the requisite common gas concentration levels. In such systems, the exchange door is located near the container differential air pressure caused by the fan that blows temperature conditioned air over the cargo. This differential pressure draws the ambient air into the container. This conventional method of achieving air exchange in a container conditioned space does not take into account changes to the conditioned space air during shipment. Moreover, such conventional system cannot quickly adjust door position as a result of changes in the composition of the conditioned space air and as a result of changes in the ambient air composition.
Other problems do exist with the manually adjusted fresh air exchange doors. One common problem is human error. If the door is forgotten or misadjusted by the container operator, the perishable cargo will suffer degradation. The containers are frequently placed in difficult to access locations on the ships and as a result, are inspected infrequently by a container operator. By the time the operator realizes the load is degrading due to harmful gas concentration levels, it is too late to manually reposition the door to achieve the concentration level required to save the cargo.
Another problem with conventional manually adjusted doors is illustrated when warm perishable cargo is loaded in warm ambient conditions. In this situation the combination of the cargo""s warm temperature and the warm ambient air entering the container through the open door results in a very long time period for temperature to pull down in the container. The consequence of the long container pull down period is cargo degradation.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.
In one aspect of the present invention, this is accomplished by providing an automatic fresh air control system and method that overcomes the shortcomings of conventional manually actuated air exchange doors by the improvements generally set forth below.
An effective improvement can be made to the manual fresh air exchange door by automating the opening of the door. The movement of the door may be controlled by a motor that in turn is controlled by a microprocessor. Such a system would be customer controlled and the customer would enter an air exchange set point in cubic feet per minute, percent door open, or cubic meters per hour into the container controller. The microprocessor system then directs the door to open based on the customer input or based on the default setting if the customer forgets to enter a value. Human error is eliminated.
Further improvements arc realized by noting the differential air pressure source varies based on the varying speed of the fan that blows temperature controlled air. The microprocessor system can utilize fan speed information of this variable speed fan to more accurately calculate the required door position to provide the desired fresh air exchange rate.
Further improvements are realized by noting that when warm cargo material is loaded, field heat may be more rapidly removed from the load if the fresh air exchange door remains closed during the temperature pull down to prevent introducing high ambient temperature air into the container. A closed door time limit must be implemented to prevent the door from remaining closed for too long of a period which might allow gas concentration levels to damage the load. The time limit is selectable by the customer and also has a default setting if the customer does not enter a time limit.
Further improvements are realized by incorporating a carbon dioxide sensor to monitor carbon dioxide levels within the container. The carbon dioxide levels are compared to a carbon dioxide set point to allow the microprocessor system to incrementally open or close the fresh air exchange door as needed to prevent the carbon dioxide levels from exceeding set point. This effectively limits the maximum concentration of carbon dioxide levels within the container. An oxygen sensor for monitoring oxygen levels may also be used.
Further improvements can be realized by utilizing the temperature set point of the load to automatically modify the carbon dioxide set point. The algorithm is a conversion from temperature set point to carbon dioxide set point as a percentage of gas concentration. The formula is a polynomial equation where the carbon dioxide set point equals various orders of the temperature set point with individual coefficients.
Further improvements can be realized by incorporating customer overrides for carbon dioxide set point and fresh air exchange door position. These overrides are useful when introducing a known concentration of gasses to the container, when hauling fresh fish or meats or other non-plant perishable produce, and when performing pre-trip functions to verify the health of the system that controls the temperature and atmosphere of the conditioned space.